1. Field of the Invention
The present invention relates to processes for preparing calcium carbonate hydroxodialuminates with a hexagonal platelet-shaped crystal habit, to the aluminates thus obtained in compositions and stabilizer systems, and to the use thereof.
2. Description of the Background Art
Novel lead-free PVC stabilizers require hydrotalcites as a stabilizer component for long-term stabilization in rigid PVC applications.
EP-A 930 332 also describes the use of tricalcium hydroxodialuminate for zinc-free systems. Calcium carbonate hydroxodialuminates are not described herein.
WO-A 93/25613 describes the preparation of katoites. Carbonate-containing compounds are not mentioned. The synthesis of katoites proceeding from calcium hydroxide and aluminum hydroxide is also described in DE-A 2 424 763. The flame-retarding action in thermoplastics, especially polystyrene, HDPE, PVC, SBR and EVA, is also demonstrated herein.
WO-A 92/13914 claims hydrocalumites as PVC stabilizers. Carbonate-containing homologs are not mentioned here either. The synthesis is effected by the coprecipitation method, which leads to additional salt ballasts in the wastewater.
DE-A 1 952 6370 discloses mixed alkaline earth metal aluminum hydroxides as acid scavengers for processing of thermoplastics. In the course of the preparation, molar amounts of NaOH have to be disposed of here via the salt burden.
Tetracalcium monocarbonate dodecahydroxodialuminate was isolated and characterized for the first time by Turriziani & Schippa [Ric. Sci. 26, 2792, (1956)]. The synthesis is costly and inconvenient (leaching of cement pastes) and the yield is low (preparation of thin films). In earlier studies, tetracalcium tetradecahydroxodialuminates were described, in which carbonates (hemi- or tetracarbonates) were later detected, though the carbonate formation very probably proceeded with carbon dioxide from the air.
JPCA Res. & Devel. Lab. (Portland Cement Assoc.) 4, 2 (1962) describes a method of forming Ca4Al2(OH)13(CO3)0.5*8H2O in the course of reaction of Ca3Al2O6 with Ca(OH)2 in water, the product forming after a 6-month reaction time through a side reaction with the CO2 from the air.
Act. Cryst. C54, 1214 (1998) describes Ca4Al2(OH)12CO3*5H2O in a three-component solid-state synthesis proceeding from Ca(OH)2, Al(OH)3 and CaCO3 at 120° C. and a pressure of 2 kbar. The reaction time is 4 weeks. This study also includes an X-ray structure analysis.
A similar process is described in Cem. Conc. Res. 29, 63 (1999). A further synthesis is published in J. Res. NBS (National Bureau of Standards) 64A, 333 (1960): wherein monocalcium aluminate—obtained by burning Al(OH)3 and CaCO3 at 1250° C. for several hours—is mixed with Ca(OH)2 and Na2CO3.
Recently, a practicable method for preparing tetracalcium monocarbonate dodecahydroxodialuminate hydrate has been specified: calcium nitrate is reacted with aluminum nitrate in the presence of sodium hydroxide solution and sodium carbonate solution in one step, the resulting gel being aged at 65° C. for 24 hours [J. Mater. Chem. 6 (1), 103 (1996) and Chem. Mater. 13, 3507 (2001)]. However, this coprecipitation process is not employable industrially (expensive reactants) and not very environmentally friendly, since considerable salt burdens are obtained in the wastewater.
None of these synthesis methods have any industrial significance whatsoever, since there are reaction times of several weeks, temperatures of more than 1000° C. or a low yield, such that the majority can only be referred to as methods of formation.
There is therefore a need for alternative processes for preparing calcium carbonate hydroxodialuminates and for further possible uses of such products.